Process for the production of staff panels according to a semidry method and installation for implementing the process

ABSTRACT

Production of plasterboard includes spreading of a mixture formed of  gyps fibers and water onto a continuously moving belt to form a spread layer subsequently precompressed to 110% to 180% of a final board thickness and successively wetted from both sides and thereafter compressing the wetted layer to the final board thickness.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase of PCT/EP 92/02055 filed Sep. 4,1992 and based, in turn, on German national application P41 29 466.1 ofSep. 5, 1991 under the International Convention.

This application is copending with Ser. No. 07/972,493 which has sinceissued as U.S. Pat. No. 5,368,663 based upon PCT/EP91/01359 filed Jul.19, 1991.

FIELD OF THE INVENTION

The present invention relates to a process and an installation forproducing plasterboard.

BACKGROUND OF THE INVENTION

In order to produce staff panels according to a dispersion method, amixture of binding agents, reinforcement materials and optionallyfillers is spread, mostly in several layers, upon a forming belt whichmoves continuously underneath dispersion devices and is subsequentlycompressed in a press. As binders gypsum or gypsum mixtures are used,e.g. hemihydrate, dihydrate, cement and additives, such as settingaccelerants and/or retardants. As reinforcement materials, fibers, e.g.of waste paper, and as fillers e.g. perlite are used. The additives canfirst be mixed with the water and then added with the water to theplaster mixture.

During these process steps the gypsum has to be supplied with the waterrequired for setting. It has been found that this is one of the mostdifficult tasks of the entire process.

In a process known from DE 27 51 466 and DE 27 51 473 the mixture isspread in a dry state and sprayed with water from above upstream of thepress, whereby the water absorption is assisted by the vacuum appliedunder the permeable support belt. The dispersed layer can then absorb anexcess of water, which is squeezed out in the press.

The disadvantages of this procedure include the need for a dewateringpress. The resulting squeezed out water contains gypsum and there is ahigh residual wetness of the pressed plate which has to be dried.

By contrast in a so-called semidry process known from EP 0 153 588 onlyso much water is supplied as can be absorbed by the dry mass of gypsum,fibers and optional fillers, i.e. only so much water that during thepressing of the layer no water is squeezed out.

As is known from EP 0 153 588, the water is supplied in two fractions.The first fraction of water is supplied during the preparation of thespreadable mixture with a dry mix of gypsum and fibers. The secondfraction of water is sprayed onto the individual layers, immediatelyafter the spreading of the layer. Subsequently the wetted layer iscompressed in a preliminary press and then compressed to its finaldensity in a main press.

In order to improve this process it is known from DE 38 01 315 to let anupper screen belt run along in the presses during compression, for thepurpose of venting. Further it is known from DE 39 06 009 to introducethe first water fraction with wetted and ground paper fibers. This way amixer for the preparation of the dry mix can be eliminated. Thespreadable mix is thereby produced by mixing the gypsum and the fibersprovided with the first water fraction.

However in this process a considerable amount of water, the second waterfraction, has to be sprayed. The spraying of spread layer or layers hasa few disadvantages.

The loose, spread material can not absorb the water well enough. As aresult in the upper areas there is excess water and corresponding lowerareas with less water, which means that the wetting achieved by thisspraying is uneven. The wetting evens out only during compression in thepresses.

Since a water excess has an accelerating effect on the setting of thegypsum, the uneven wetting leads to different setting speeds in thespread, wetted layer.

There is also the danger that especially the bottom side of the firstlayer may receive too little water for the complete setting of thegypsum.

A further disadvantage is that devices for the spraying of the layersare arranged between the dispersion devices. In addition to thedisadvantageous space requirements of the spraying devices, thisarrangement requires that the various strata of the layer traversedifferent stretches until they reach the presses, thereby being wettedat different times. This results in differences in the onset of thesetting process of the gypsum in the various strata.

Just like the different setting speeds due to uneven wetting in a layer,the different onset times of the setting in the various strata lead toweak areas in the finished plate caused by uneven setting. The controlof the production process is made more difficult, especially since thereis no possibility to monitor the setting process in the dispersed,wetted layer and to correct it.

DE-OS 40 25 797 describes a process wherein a portion of the secondfraction of the water is supplied after compression by preliminary andmain presses. The water is first applied by spraying to the bottom sideof the plate and subsequently to its top side.

Subsequently the after-wetted plate is additionally compressed with apressure which is not higher than that of the main press. In this casean additional press is needed. Besides only a small portion of thesecond water fraction can be supplied to a compressed plate during theadditional wetting. A spraying of the spread material with thepossibility of uneven wetting remains.

OBJECTS OF THE INVENTION

It is the principal object of the present invention to provide animproved process for making plasterboard in which a more uniform wettingof the plasterboard layer is achieved at low cost.

Another object is to provide an improved apparatus for makingplasterboard by carrying out the improved method.

SUMMARY OF THE INVENTION

These objects are achieved in its method aspect by the steps of:

(a) forming a mixture of gypsum, fibers and water in an amount less thannecessary for hardening of the gypsum;

(b) spreading the mixture onto a continuously moving belt to form aspread layer;

(c) precompressing the layer to 110% to 180% of a final board thickness;

(d) at least upon precompression successively wetting the layer fromboth sides; and

(e) thereafter compressing the wetted layer to the final boardthickness.

According to the apparatus aspects of the invention, the apparatus cancomprise:

means for mixing gypsum, fibers and water in an amount less thanrequired for hardening of the gypsum to form a mixture;

at least one forming belt;

means juxtaposed with the belt and receiving the mixture for spreadingthe mixture in at least one layer on the belt;

means forming a preliminary press along the belt downstream of the meansfor spreading, for compressing the belt to 110 to 180% of a finalthickness of the plasterboard;

means downstream of the means for spreading for successively applyingwater to upper and lower parts of a layer of the mixture precompressedon the belt and including upper and lower wetting devices;

a main press downstream of the preliminary press along the belt forpressing the layer after wetting of upper and lower parts thereof to afinal plasterboard thickness;

an upper screen belt guided through the preliminary press and the lowerwetting device;

a lower screen belt above the forming belt, the forming belt beingreturned downstream of the preliminary press and the lower screen beltextending at least through the lower wetting device.

In a process according to the invention the spread layer is subjected toa preliminary compression to 110 to 180% of the plate thickness, i.e.the finished size of the plate, prior to wetting with the secondfraction of water. During the preliminary compression or immediatelyafter that both sides of the layer are successively wetted. Only afterthe wetting with the entire second fraction of water is theprecompressed plate further compressed to plate thickness.

Due to the preliminary compression to 110 to 180% of final thickness alayer is created which can absorb water considerably better than thespread layer, but also better than the layer already compressed to platethickness. A particularly even wetting over the entire layer height isachieved upon a preliminary compression to 110 to 150% of final platethickness.

Particularly wet or dry areas in the dispersed layer and thereby areaswherein the setting of the gypsum is accelerated by water excess ordelayed by too little water, are avoided.

Since, in addition, the wetting of both sides of the layer takes placein immediate succession, i.e. the setting in the board startsapproximately at the same time, with the process of the invention aconsiderably more uniform setting is achieved. This leads to an improvedplate quality, especially to higher strength.

In the process of the invention it is no longer necessary to spray theindividual strata directly after spreading. The wetting devices, whichhere are arranged in or immediately downstream of the preliminary pressfor preliminary compression, are no longer contaminated by whirled-uploose material.

In addition wetting devices between the dispersion devices and theadditional press become superfluous. With the process of the inventionit is possible to obtain a plate of improved quality at lower cost.

According to a feature of the invention, the initial member containswater in an amount of 15 to 27%, preferably 18 to 24% of the dry massand, during the subsequent wetting, water in an amount of 10 to 30%,preferably 15 to 25% of the dry mass is supplied, whereby the totalwater amounts to 35 to 45% of the dry mass.

Thus a total water amount of 35 to 45% of the dry mass can be supplied.This total water amount which is approximately 2 to 3 times thestoichiometric water amount required for the setting of the gypsum,makes possible a more even wetting of the precompressed layer, withoutsqueezing out of water during the compression to final plate thickness.

Since fibers and optional fillers also absorb water, more than thestoichiometric water amount is required in order to insure a completesetting of the gypsum. A mixture to be spread containing water in anamount of 15 to 27% in relation to the dry mass contains on the one handa high proportion of the total amount of water to be supplied, while onthe other hand it still has good spreadability. A larger amount of waterwould lead to granulation or lump formation in the mixture. Mixtureswith a water amount of 18 to 24% in relation to the dry mass areparticularly suitable, i.e. have particularly good spreadability.

Advantageously, during preliminary compression the top side of the layeris wetted and after the preliminary compression the bottom side iswetted. The wetting during the preliminary compression leads to aparticularly good water absorption of the layer. The water absorptioncan be further improved by alternating preliminary compression andwetting, e.g. with the assistance of a roller press and of wettingdevices arranged between the press cylinders. Thereby the spring-back ofthe layer after compression and during wetting causes a suction effectwhich assists the water absorption of the layer.

In a further process variant the wetting of one or both sides of thelayer is done by spraying. It is possible to regulate the supplied wateramount by differential measurements of the supplied and captured runoffwater amounts, which results in a precise dosage of the supplied wateramounts. This is of particular advantage in the spraying of the bottomside of the layer, since from the bottom side the water can easily dripoff.

The bottom side of the layer can also be wetted by guiding theprecompressed layer through a water bath. The precompressed layer isguided through the water bath so that only the bottom side of the layeris wetted. For this purpose it is necessary for instance to prevent thatthe water penetrates the layer at the two edges of the layer and flowsonto the surface of the layer, by means of a screen belt with elevatedborders which are not water permeable. The water bath is supplied withthe dosed amount for wetting the bottom side of the layer.

The invention takes advantage of a self-regulating mechanism of waterabsorption of the layer. The layer is guided at a small angle into thewater bath and back again. Its water absorption increases steadily withthe wetted surface and with the height of the water level.

If a dosed amount of water is supplied to the water bath, a water levelis set wherein the supply and the absorption by the layer are balanced.The layer always absorbs the supplied amount of water, independently ofthe way this water absorption takes place in detail.

Due to the fact that thicker parts of the layer are exposed to the waterfor a longer time and therefore absorb more water, the material isevenly wetted even when the thickness of the layer varies slightly.

When underpressure (subatmospheric pressure or suction) or overpressure(superatmospheric pressure) is applied in at least one zone of the topside of the layer, the even wetting of the layer from underneath isimproved. Preferably in a first zone underpressure is applied in orderto remove the air from the pores of the layer and the screen belts. Ifthere are several zones, it is preferred to subsequently applyoverpressure and underpressure in an alternating manner.

According to the invention, the preliminary press has upper and lowerpressure rollers arranged at a distance from one another and the upperwetting device has spray nozzles between the upper pressure rollers.

The additional shaping of the bottom side of the boxes or trays or thearrangement of the guide rollers so that they create a concave glidingsurface in the travel direction makes possible a good guidance of thelayer through the vat.

A large depth of the trays (a multiple of the board thickness) andguidance mechanisms for this depth prevent turbulence in the vat whichcould be produced by the layer traversing the vat.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become morereadily apparent from the following description, reference being made tothe accompanying drawing in which:

FIG. 1 is a flow diagram showing an installation according to the stateof the art;

FIGS. 2, 3, and 4 are diagrammatic side elevational views of theapparatus for carrying out Examples 1, 2, 3 and 4 of the invention;

FIG. 5 is a view similar to FIG. 4 but which shows a tray for wettingthe bottom side of the layer of Example 4;

FIG. 6 is a detail vertical section of the tray;

FIG. 7 is an enlarged detail of a portion of the tray of FIG. 6; and

FIG. 8 is a section along line IX--IX of FIG. 6.

SPECIFIC DESCRIPTION

A known installation for the production of staff panels according to asemidry process has storage facilities for water, optionally additives,gypsum, fibers and optionally fillers. The storage facility for fibershas a conveyor coming from the paper container via a shredding device.Supply lines from the storage facility for fiber and for gypsum end in adry mixer.

The installation has two mixers 1a, 1b, fed by a supply line from thestorage facility for water and the dry mixer, via a dosage device.Optionally a further supply line leads from the storage facility forfillers via a dosage device to the second mixer 1b. The mixers 1a, 1bfor instance can be continuous horizontal mixers with mixing rotors orrotary kiln mixers.

Further the installation has three spreading devices 2a, 2b, 2c whichare arranged above a forming belt 3, one after the other in its traveldirection. The outlet of the first mixer 1a is connected via conveyorbelts with the first and third spreading devices 2a, 2c, and the outletof the second mixer 1b is connected via a conveyor belt to the secondspreading device 2b. Downstream of the first spreading device 2a andupstream of the second spreading device 2b, there is a spraying device4a with nozzles directed towards the forming belt 3.

A further spraying device 4b is located between the second and thirdspreading devices 2b, 2c and a third spraying device 4c downstream ofthe spreader 2c.

Downstream of the third spraying device 4c a preliminary press 5, e.g. aroller press, is provided. The forming belt 3 is guided over the lowerrollers and a screen belt 6 is guided over the upper rollers of thepreliminary press 5. The forming belt 3 and the screen belt 6 are bothalso guided through a main press 7, also a roller press, which isarranged downstream of the preliminary press 5. The main press 7 hasadditional steel belts 8 guided within the forming belt 3 and within thescreen belt 6.

The screen belt 6 extends over a vacuum box 10 connected with a blower 9and arranged downstream of the main press 7 at the level of the upperrollers and is guided in reverse downstream of the box. The forming belt3 projects under the vacuum box 10 and is guided in reverse in its firstquarter. It is an endless plastic belt.

In the last quarter of the vacuum box 10 a further lower belt 11 starts,so that underneath the vacuum box 10, between the forming belt 3 and thelower belt 11 there is a free stretch. On this stretch a lower sprayingdevice 12 with nozzles for spraying the bottom side of the layer isarranged.

An upper spraying device 13 with nozzles is located immediately afterthe return point of the screen belt 6, above the lower belt 11.

The lower belt 11 is guided through an additional press 14, i.e. aroller press with an upper belt succeeding the upper additional wettingdevice 13, and through further devices e.g. for drying the plate (notshown).

FIG. 1 also shows a paper treatment device 15, which besides storagefacilities for additives, paper and water, has a mixer to which leadsupply lines from the storage facilities via dosage devices.

In the supply line coming to the mixer from the storage facility forpaper, between the storage facility and the dosage device, a preliminaryshredder is arranged. The outlet of the mixer is optionally connectedwith the storage facility for fibers via a dosage device and a millingdevice. When the paper treatment device 15 is provided, the storagefacilities for water and the dry mixer are no longer necessary.

In operation a mixture of gypsum, fibers and the first fraction of thewater is spread by the devices 2a, 2b, 2c in three layers onto thecontinuously moving forming belt 3. Each layer is wetted directly afterspreading by the spraying devices 4a, 4b, 4c with a portion of thesecond water fraction.

The mixer 1a feeds the first and third dispersion devices 2a and 2c,which produce the bottom and top layers, i.e. the outer parts of thespread material. In addition the second mixer 1b which feeds the seconddispersion device 2b can be supplied with fillers. This way it ispossible to select a different composition for the intermediate strataof the dispersed layer.

The dispersed and wetted layer is compressed to a final thickness firstin the preliminary press 5 and subsequently in the main press 7. Afterthat the resulting plate is additionally sprayed in succession frombelow and from above with the balance of the second water fraction.During the additional wetting from underneath, the plate is held by thevacuum box 10. The additionally wetted plate is recompressed in theadditional press 14.

When the paper treatment device 15 is used, the paper is preshredded andmixed with the first water fraction, i.e. up to 200% per dry mass ofpapers, and optionally with additives. After a certain dwelling time theprewetted paper is milled to fibers in the milling device and conveyedto the storage facility for fibers.

In the two mixers 1a, 1b the wetted fibers are mixed with gypsum and themixture is then supplied to the dispersion devices 2a, 2b, 2c.

DESCRIPTION OF THE INVENTION EXAMPLE 1

An installation according to the invention (FIG. 2) distinguishes itselfover the state of the art due to the following features. It has anadditional lower screen belt 16. The lower screen belt 16 is locatedunderneath the spreading devices 2a, 2b, 2c arranged immediately oneafter the other and in the preliminary press 5 on the forming belt 3,which is returned downstream of the preliminary press 5. The formingbelt 3 can thereby be replaced by a sliding table. The lower screen belt16 extends all the way through the main press 7 where it is located onthe lower steel belt 8.

The preliminary press 5 and the main press 7 are arranged at a distancefrom each other.

Downstream of the preliminary press 5 and upstream of the main press 7,under the screen belt 16 a lower spraying device 18 provided with adosage device 17 is arranged. Above the upper screen belt 6 runningthrough the preliminary press 5 and the main press 7, opposite to thelower spraying device 18, there is a vacuum box 20 connected to a blower19. Downstream thereof, also before the main press 7, above the upperscreen belt 6, an upper spraying device 22 also provided with a dosagedevice 21 is arranged. Underneath the lower screen belt 16, opposite theupper spraying device 22, there is a further vacuum box 24 connectedwith a blower 23.

By contrast to an installation of the state of the art as in FIG. 1, theinstallation of the invention has no spraying devices 4a, 4b, 4c betweenthe dispersion devices 2a, 2b, 2c and no additional press 14.

In operation in an installation with a production width of 2,500 mm anda belt speed of 15 m/min the following amounts are dosed into the drymixer:

    ______________________________________                                        Gypsum hemihydrate (plaster of Paris)                                                                 20,500  kg/h                                          Gypsum dihydrate (milled)                                                                             100     kg/h                                          Paper fiber (dry)       4,200   kg.h                                          ______________________________________                                    

Each of the mixers 1a, 1b are supplied with 50% of the dry mass of24,800 kg/h. In addition 2,580 l/h of water, i.e. 20.8% of the dry massare supplied to the first mixer 1a, and to the second mixer 1a 2,880 l/hwater, i.e. 23.2% of the dry mass are introduced, with the proportionsof retardants and accelerants adjusted to the gypsum.

The mixture is spread on the forming belt 3 in three strata. Thedispersed layer has a height of approximately 50 mm and is precompressedin the preliminary press 5 to a height of approximately 12 mm, i.e. toapproximately 120% of the plate thickness. It springs back toapproximately 16 mm. After that with the spraying devices 18, 22 thelayer is sprayed first from underneath and subsequently from above eachtime with 2,280 l/h of water, i.e. a total of 18% of the dry mass.During spraying the supplied water amount is adjusted based ondifferential measurements of the supplied and discharged water amounts.A total water amount representing approximately 40% of the dry mass,i.e. approximately three times the stoichiometric water amount, issupplied.

During spraying air is evacuated from the layer and from the screenbelts 16, 6, by the vacuum boxes 20, 24 [sic] arranged opposite to thespraying devices 18, 22.

In the main press 7 the layer is compressed to a plate thickness of 10.3mm. After setting the raw plate still contains 14.2% residual wetness.After drying a plate is obtained with a density of 1,150 kg/m3 and abending resistance of 8.0N/mm.

EXAMPLE 2

In the installation of Example 2 (FIG. 3), the upper screen belt 6 isguided only through the main press 7. A second upper screen belt 25extends through the preliminary press 5, along the vacuum box 20opposite the lower spraying device 18 and along a further transfervacuum box 27 connected with a blower 26. Downstream of the transfervacuum box 27 the upper screen belt 25 reverses its path. The upperspraying device 22 is arranged between the return point of the upperscreen belt 25 and the main press 7.

The lower screen belt 16 runs through the preliminary press 5, along thelower spraying device 18 and over one or more relief nozzles 29connected with a blower.

The relief nozzles 29 are arranged at the end of vacuum box 20 [sic].The lower screen belt 16 projects up to the front edge--considered intravel direction--of the transfer vacuum box 27 and is returned fromthere.

A further smooth, lower belt 30 starts at the rear edge of the transfervacuum box 27, extends underneath the upper spraying device 22 and isguided through the main press 7. There is no vacuum box arrangedcorrespondingly opposite the upper spraying device 22.

In operation the layer precompressed in the preliminary press 5 is firstwetted on its bottom side by the spraying device 18. Subsequently thelayer is detached from the lower screen belt 16 by blowing air throughthe detaching nozzles 29 and transferred to the smooth lower belt 30with the assistance of the transfer vacuum box 27. There the layer iswetted on its top side by the spraying device 22 and compressed to finalplate thickness in the main press 7. Thereby the bottom side of theplate is smoothed by the smooth, lower belt 30.

EXAMPLE 3

The installation of the Example 3 (FIG. 4) differs from that of Example2 in that the lower screen belt 16, as in Example 1, is guided all theway through the main press 7. Correspondingly a vacuum box 24 withblower 23 is arranged opposite to the upper spraying device 22 locateddirectly upstream of the main press 7.

The installation of Example 3 differs from the one in Example 2 also inits lower wetting device. The lower wetting device has a tray or trough32 provided with a dosage device 31. Outer rollers 33 are arranged aboveeach the front and rear edges of the tray 32 considered in traveldirection. Between the outer guide rollers 33 extends a box divided intwo in the direction of travel with venting holes on its bottom side, onwhose front half a suction blower 35 and on whose rear half a pressureblower 36 are connected. The bottom side of the box 34 projecting intothe tray 32 is designed as a sliding surface, concavely curved in thetravel direction. The curvature corresponds approximately to a radius of5 to 15 m. Thereby it is important that the outer guide rollers 33 andthe box 34 be arranged so that screen belts 16, 25 are guided downwardsand back again at a small angle.

The upper screen belt 25 is returned downstream of tray 32. At thispoint the lower screen belt 16 is guided over a further roller 37.

In operation the layer precompressed in the preliminary press 5 isguided through tray 32 so that only the bottom side of the layer iswetted. From the top side next in a first zone air is evacuated from thelayer and the screen belts 16, 25 and subsequently in a second zone anoverpressure is applied to the upper side of the layer. The respectivepressures are adjustable.

In the tray 32 a precisely metered water amount is introduced, which isabsorbed by the layer. Subsequently the top side of the layer is wettedby spraying devices 22 and finally the layer is compressed in the mainpress 7 to plate thickness.

EXAMPLE 4

In the installation of Example 4 (FIG. 5) the upper screen belt 6 andthe lower screen belt 16 are guided through the preliminary press 5,wherein the upper spraying device 22 is arranged, the lower wettingdevice and the main press 7. Thereby upper and lower pressure rollers 38of the preliminary press 5 are arranged at a distance from each other.Between the upper pressure rollers 38 there are nozzles 39 of the upperspraying device 22.

The lower wetting device consists of tray 32 provided with a dosagedevice 31 as described in Example 3. Between the outer guide rollers 33arranged above the edges of tray 32, suction/pressure boxes 40 andfurther guide rollers 41 are alternately arranged in succession. Theboxes 40, four in this example, are connected via ducts 42 with thesuction blower 35 and the pressure blower 36. In the ducts 42 there arevalves 43, so that the blowers 35, 36 can be connected selectively withthe boxes 40. The air pressure is adjustable.

As in Example 3, the guide rollers 41 form a sliding surface concavelycurved in the travel direction, along which the upper and the lowerscreen belts 25, 16 are guided through the tray 32. The dipping depth ofthe lower screen belt 16 is of the order of magnitude of the layerthickness.

The extent of tray 32 in the travel direction corresponds approximatelyto that of the main press 7, e.g. 3 to 5 m. It is wider than the layeron both sides by approximately 25 cm. Its depth is about 10 times theplate thickness.

Underneath the lower screen belt 16 on the tray 32, there are guidingdevices 44, 45 extending in the travel direction, with vertical andhorizontal guide surfaces as is illustrated in FIG. 7.

The lower screen belt 16 is wider than the upper screen belt 6(respectively upper screen belt 25, (FIG. 4). Its lateral edges aresealed with elastic plastic material 46 as is clearly shown in FIG. 8.The cover projects on both sides by about 5 cm into the area of theprecompressed layer as is illustrated in a greater detail in FIG. 8.Besides in the tray 32 the lower screen belt 16 is guided by its edgesover rollers 47. The rollers 47 are arranged so that the edges of thelower screen belt 16 are bent upwards. As is shown in a greater detailin FIG. 6 tray 32 is provided with an inlet 48 at its front end. Theinlet 48 is designed in the manner of a diffuser with an overflow (notshown), extending over the entire width of the vat. FIG. 6 illustratesthe dosage pump 31 generates a flow and 31 is connected to the inlet 48via a flow meter 49.

In operation the dispersed layer is alternately compressed in thepreliminary press 5 by pressure rollers 38 (FIG. 5) and sprayed in thearea between the pressure rollers 38. Thereby the spring-back of thelayer when it is no longer subjected to the load of the pressure rollers38 results in an enhanced suction of the water in the layer.

Subsequently the precompressed layer arranged between the belts 16 seenin FIGS. 2-5, 25 is guided through a water bath, namely through the vat32 supplied with a metered amount of water, so that only the bottom sideof the layer is wetted. This is insured by the upturned edges of thelower screen belt 16 which are sealed with the plastic material 46. Thelayer is guided into the water bath and then back at a small angle, e.g.of 1 to 10 degrees seen in FIG. 7 which is a front view of the traysillustrated in FIGS. 5 and 6 and is particularly well illustrated inFIG. 8 which shows the guide roller 47 in a greater detail. Due to thedescribed self-regulating mechanism it absorbs exactly the apportionedamount of water.

In this example the layer is alternately loaded and relieved by theguide rollers 38, 41 (FIGS. 5 and 6).

The spring-back of the layer between the guide rollers 38, 41, enhancesthe wetting of the layer through aspiration of water as in the upperspraying device 22 of this example. This spring-back of the layer duringwetting can also be achieved by guiding the layer in the water bathalong a curved surface, whose radius increases constantly.

By means of the suction/pressure boxes 40 (FIGS. 5 and 6) between theguide rollers 41 the air is evacuated from the layer in a first zone, inorder to remove the air from the layer and most of all from the lowerscreen belt 16. Then in a second zone at the rear box a certain airpressure from above is applied to the layer guided through the waterbath. This applied air pressure counters the effect of uneven wetting ofareas with variable density of the layer. The boxes 40 can also beoperated in such a way that through the boxes 40 alternately in one zoneair can be aspired by one box 40, and by the next one in the followingzone air can be applied, whereby in the first box 40, i.e. in the firstzone, air is always aspired.

In an installation with a production width of 2,500 mm and a belt speedof 15 m/min the following dosed amounts are supplied to each of themixers 1a and 1b:

    ______________________________________                                        Gypsum hemihydrate (plaster of Paris; 5.9% H2O)                                                         10,500  kg/h                                        Gypsum dihydrate (milled) 50      kg/h                                        Paper fibers (dry weight) 2,000   kg/h                                        Water (contained in paper fibers)                                                                       2,800   kg/h                                        ______________________________________                                    

The paper fibers saturated with water are prepared in the papertreatment device 15. The 30,700 kg/h of mixture of gypsum, fiber andwater, whereby the water amount represents approximately 22% of the drymass, is spread in three strata. The dispersion height is about 45 mm.The layer is precompressed in the preliminary press 5 to a thickness of11.5 mm, i.e. approximately 112% of the plate thickness. It springs backto 15 mm. During precompression 2,400 l/h water are sprayed from above.In the vat the layer absorb from underneath also 2,400 l/h water.

Therefore during wetting a water amount representing 19% of the dry massis supplied. This way the total amount of water supplied representsapproximately 41% of the dry mass and approximately three times thestoichiometric water amount.

In the main press 7 the layer is compressed to a plate thickness of 10.3mm. After setting the raw plate still contains 14.2% residual wetness.After drying a plate with a density of 1,170 kg/m3 and a bendingresistance of 9.2N/mm is obtained.

I claim:
 1. A process for producing plasterboard which comprises thesteps of:(a) forming a mixture of gypsum, fibers and water in an amountless than necessary for hardening; (b) spreading said mixture onto acontinuously moving belt to form a spread layer; (c) precompressing saidlayer to 110% to 180% of a final plasterboard thickness; (d) at leastupon precompression successively wetting said layer from a top side anda bottom side; and (e) thereafter compressing the wetted layer to saidfinal plasterboard thickness.
 2. The process defined in claim 1 whereinsaid layer is precompressed to 110% to 150% of the final plaster boardthickness in step (c).
 3. The process defined in claim 2 wherein saidmixture is formed in step (a) as an amount of water equal to 15 to 27%of the dry mass of said mixture, and wherein in step (d) an amount ofwater equal to 10 to 30% of said dry mass is supplied whereby the totalamount of water in said layer upon compression to said final thicknessin step (e) is equal to 35 to 45% of the dry mass.
 4. The processdefined in claim 3 wherein in step (a) water is present in said mixturein an amount of 18 to 24% of said dry mass thereof and in step (d) wateris added in an amount of 15 to 25% of said dry mass to said layer. 5.The process defined in claim 4 wherein in step (d) said top side of saidlayer is wetted during precompression and said bottom side of said layeris wetted after precompression.
 6. The process defined in claim 4wherein said wetting in step (d) is carried out by spreading water ontosaid layer and the amount of water supplied to said layer is controlledby monitoring quantities of supplied water and water lost from saidlayer.
 7. The process defined in claim 4 wherein said layer is wetted instep (d) by guiding said layer into a water bath so that only a bottomportion of said layer is wetted and supplying water to said water bathin a metered quantity for wetting said bottom portion.
 8. The processdefined in claim 4, further comprising the step of applying to at leastone zone of said top side of said layer a subatmospheric pressure. 9.The process defined in claim 4, further comprising the step of applyingto at least one zone of said top side of said layer a superatmosphericpressure.
 10. An apparatus for producing plasterboard, comprising:meansfor mixing gypsum, fibers and water in an amount less than required forhardening of said gypsum to form a mixture; at least one forming belt;means juxtaposed with said belt and receiving said mixture for spreadingsaid mixture in at least one layer on said belt; means forming apreliminary press along said belt downstream of said means forspreading, for compressing said spread layer to 110 to 180% of a finalthickness of said plasterboard; means downstream of said means forspreading for successively applying water to upper and lower parts of alayer of said mixture precompressed on said belt and including upper andlower wetting devices; a main press downstream of said preliminary pressalong said belt for pressing said layer after wetting of upper and lowerparts thereof to a final plasterboard thickness; an upper screen beltguided through said preliminary press and said lower wetting device; alower screen belt above said forming belt, said forming belt beingreturned downstream of said preliminary press and said lower screen beltextending at least through said lower wetting device.
 11. The apparatusdefined in claim 10 wherein said preliminary press has upper and lowerpressure rollers spaced apart from one another and said upper wettingdevice includes spray nozzles arranged between said upper pressurerollers.
 12. The apparatus defined in claim 10 wherein said lowerwetting device includes a tray through which said layer is passed andguides for directing said screen belts through said tray, edges of saidlower screen belt being turned upwardly and being water impermeable. 13.The apparatus defined in claim 12 wherein above said tray a box isprovided for selectively applying fluid pressure to said layer, anunderside of said box having a concave curved surface for guiding saidlayer into said tray.
 14. The apparatus defined in claim 12, furthercomprising guide rollers above said tray forming a convex guide path ina travel direction for said layer.
 15. The apparatus defined in claim 12wherein said tray has a depth which is a multiple of said finalthickness of said plasterboard.